Study On The Preparation And Performance Of LiNiO₂ As Cathode Materials For Lithiun Ion Batteries And Its Precursor NiOOH

Among the cathode materials of rechargeable lithium batteries, LiCoO has been commercially used as positive active material for lithium-ion cells, but it has some disadvantages such as low effective specific capacity, toxicity and expensiveness. LiNiO has been regarded as the most potential cathode material for lithium-ion batteries because of its high discharge capacity, low cost and environmentally benign. However, it is difficult to obtain large batch stoichiometric LiNiO by conventional solid-phase reaction with Ni(OH) or Ni(Ⅱ) salts at high temperature. That restricts its industrialized production. Therefore, the key to the development of LiNiO cathode material is to develop a mild and controllable new synthesis technology. Based on the studies on highly pure nickel oxyhydroxides (NiOOH), the aim of this dissertation is to prepare layered LiNiO material by ion-exchange reaction in air atmosphere. In addition, we also studied various technology conditions of electro-synthesis NiOOH to provide fundamental support for its application in the synthesis of LiNiO and the manufacture of alkaline Zn/NiOOH batteries.In order to get rid of consuming too much reagent and reduce reaction steps in chemical synthesis, we report a new method to obtain highly pure NiOOH by one-step electrolysis of sphere Ni(OH) in alkali solution. The influence of various parameters (e.g. electrolyte concentration, temperature, electrolysis duration, and current density) on electrolysis was investigated. The experimental results indicate that the optimum conditions are a temperature of 30℃, a current density of 60 mA g, an anodic potential of 1．73 V-1．85 V (vs. Zn/ZnO) and an electrolysis duration of 360 min. The electrolysis-generated NiOOH has high purity of 96% and high electrochemical activity, providing a discharge capacity of over 273．7 mAh g at current density of 30 mA g.In the study of one-step electrolysis, we found that the effective electronic contact between the anode and Ni(OH) was beneficial to the improvement of the purity of NiOOH and the electrolytic efficiency. In the dissertation we firstly propose KMnO, a representative of permanganates, as the medium of electron transfer during the electrolytic process. In this process, the discharge process of single point to point contact is transformed to the process of both point to point contact and the overall surrounding contact of sphere Ni(OH) through the self redox reaction, thus making a highly efficient electron transfer process. In this dissertation, the catalytic electrolysis mechanism is discussed. The influence of various parameters (e.g. kind and concentration of catalyst, reaction temperature and electrolysis duration) on electrolysis were investigated. The results show that the NiOOH prepared by catalytic electrolysis has good electrochemical performance and displays a discharge capacity of 267 mAh g at current density of 120mA g.Based on the study of NiOOH, we developeded the new technologies for preparing LiNiO by ion exchange reaction of NiOOH to LiOH·HO with three different methods.(1) The synthesis process of LiNiO by solid phase reaction using NiOOH as raw materials instead of Ni(Ⅱ) was studied. The advantage of this method is simple in technology process and easy in industrialization. The LiNiO cathode material, which is synthesized at 650℃for 10h in air atmosphere at Li/Ni molar ratio of 1．05, has typical layered structure and almost keeps homogenous sphere shape like the raw material NiOOH. The charge/ discharge test indicates that the sample has a high initial discharge capacity of 175．9 mAh g, an initial charge-discharge efficiency of 85．3% and capacity retention of 81．3% after 20 cycles.(2) The influence of various technological conditions on the preparation of LiNiO material by hydrothermal reaction at low temperature was studied. The optimum synthesis condition is at 210℃for 72h. Charge/discharge experiment results show that the discharge capacity of the LiNiO sample is only 129．9 mAh g, much lower than the theoretical capacity of LiNiO.(3) The LiNiO sample was synthesized by ion exchange reaction using NiOOH and melted LiOH as raw materials. The LiNiO compound synthesized at 550℃for 20h was found to be the optimum preparation condition in this study. The XRD and SEM analysis results indicate that the prepared LiNiO has a layered structure and sphere shape. Charge and discharge tests show that the LiNiO cathode exhibits good electrochemical performance. The first discharge capacity of the sample is 169．1mA h g and the initial coulombic efficiency is 90．96%. Galvanostatic charge/discharge and cyclic voltammetry tests show that LiNiO electrode exhibits good cycling reversibility and the capacity retention is 91．6% after 20 cycles.Finally, the storage performance of LiNiO cathode in air atmosphere was investigated. The results show that LiNiO material can generate LiOH on the surface through hydrolytic reaction and further form a LiCO cover with the adsorption of CO. That leads to the deterioration of its electrochemical performance. Therefore, the storage stability of LiNiO is worse than LiCoO material in wet air atmosphere. It is necessary to store in sealed package to isolate the LiNiO product from HO and CO for keeping its storage performance.